1. Field Of The Invention
The invention relates to an improved piezoelectric actuator module and more particularly to such an actuator bathed by liquid media, having a sheathed piezoelectric actuator that can be used in a piezoelectric injector for metering fuel in a chronologically and quantitatively precise manner in an internal combustion engine.
2. Description of the Prior Art
One known piezoelectric injector essentially comprises a holder body and the piezoelectric actuator module, which is disposed in the holder body and has the piezoelectric actuator, located between a head and foot part and attachment components, that comprises a plurality of piezoelectric elements stacked one above the other.
It is known per se that to construct the aforementioned piezoelectric actuator, piezoelectric elements can be used in such a way that by utilizing what is known as the piezoelectric effect, control of the needle stroke of a valve or the like can be accomplished. Piezoelectric layers of the piezoelectric elements are constructed from a material having the suitable crystalline structure, such that when an external electrical voltage is applied, a mechanical reaction of the piezoelectric element ensues, which depending on the crystalline structure and on the regions contacted by the electrical voltage, represents a compression or tension in a predeterminable direction. Such piezoelectric actuators are suitable for instance for applications in which reciprocating motions take place under conditions involving strong actuation forces and high cycle frequencies.
For example, one such piezoelectric actuator, as a component of a piezoelectric injector in so-called common rail injection systems (CR injector), is known from German Patent Disclosure DE 10026005 A1. In this piezoelectric actuator as well, piezoelectric elements are arranged in a stack that is held between two stops in prestressed fashion via an actuator foot and an actuator head. Each piezoelectric layer is also fastened here between two internal electrodes, by way of which an electrical voltage can be applied from outside. Because of this electrical voltage, the piezoelectric elements then each execute short reciprocating motions in the direction of the potential drop, and these motions add up to make the total stroke of the piezoelectric actuator. This total stroke is variable by way of the magnitude of the voltage applied and can be transmitted to a mechanical final control element.
In these know CR injectors, a nozzle needle controlled indirectly by the piezoelectric actuator module is present as a fuel valve, and the piezoelectric actuator, by way of a sleeve and diaphragm arrangement as the sheath that compensates for stroke and temperature-related expansion is surrounded directly or indirectly by the pressure of the fuel, which in such low-pressure systems is approximately 60 bar, thus making it possible to achieve the opening and closing functions. To attain optimal deflection while requiring minimal space, the internal electrodes of the piezoelectric actuator must encompass the entire cross section of the piezoelectric actuator, except for the inactive contacting zone. This means that the internal electrodes of alternating polarity are exposed at the side faces of the ceramic layers.
This known concept cannot readily be adopted in CR injectors with direct control of the nozzle needle, however, since in that case the piezoelectric actuator module is operated in the high-pressure region of the fuel, and a rigid sleeve and diaphragm arrangement as the sheath, under the conditions of what space is available in the piezoelectric injector, would not withstand the high system pressures and relatively long reciprocating motions of the piezoelectric actuator.
For this reason, an electrical passivation of these side faces of the piezoelectric actuator and of the attached contact components for the electrical passivation of the entire piezoelectric actuator is necessary, to prevent the media bathing the piezoelectric actuator from getting underneath it and to avoid the attendant possible electrical sparkovers or short circuits between adjacent internal electrodes. Such effects can result for instance from moisture or from the fuels and lubricants themselves, such as diesel, rapeseed methylester, water, or the like, in the operation of the piezoelectric actuator.
To achieve an electrical and mechanical insulation of the piezoelectric actuator, a flexible sheath of the piezoelectric actuator is often proposed. From German Patent Disclosure DE 10230032 A1, to avoid the aforementioned disadvantages, an arrangement with a piezoelectric actuator module bathed in fluid media is known in which the piezoelectric elements are cast in a variable-shape insulation compound, which in turn is placed in a housing jacket that is closed off laterally and at the top and bottom ends from the medium.
However, fuel can migrate under the two ends of this housing jacket, in the region where they contact the steel attachment components of the head part and foot part of the piezoelectric actuator module, and thus the known arrangements do not offer reliable sealing protection, and hence the proper function of the piezoelectric actuator over its service life is threatened.